Preventing corrosion: paint, galvanizing and sacrificial metals

0:00How do we stop steel beams, nails and ships from rusting?

0:05Steel is mostly iron, and iron rusts when it meets oxygen and water.

0:11Salt, as in sea water, makes it rust faster.

0:15Here are three common defences: a barrier, galvanizing and a sacrificial metal.

0:23Paint on this beam is a barrier that keeps out water and oxygen.

0:28Paint, oil, grease and plastic are all barriers.

0:33Many people think a small scratch in the paint does not matter.

0:37But water and oxygen get in through the scratch, and rust spreads under the paint.

0:43So a barrier works only while it is unbroken.

0:47Galvanizing means coating steel with zinc, like this nail.

0:52The zinc is a barrier too: it keeps water and oxygen out.

0:57Zinc is also more reactive than iron: it reacts more easily.

1:02So at a scratch, the zinc corrodes first and the steel is protected.

1:08A sacrificial metal is a more reactive metal joined to steel, like zinc blocks on a ship's hull.

1:15The zinc corrodes instead of the steel hull.

1:19A buried steel pipeline can be wired to a magnesium block.

1:24The blocks slowly corrode away, so they are replaced when used up.

1:29Stainless steel resists rust another way: its chromium forms a thin protective oxide layer.

1:36So, to stop rusting, block water and oxygen, or sacrifice a more reactive metal that corrodes first.